According to "Introduction: Current Cross-References" (GURPS Thaumatology, p. 5), the book is designed to stand alone and doesn't need GURPS Magic. The book presents alternatives to the spell system provided in the Basic Set, as well as some modifications of the default spell system.
If you are using the spell-magic rules from the Basic Set, then GURPS Magic will be relevant (though not needed), but you could easily turf the generic spell system entirely and implement magic in your game world with one of the Thaumatology magic system alternatives.
I been playing GURPS since 1987 and the basic combat of attack, defend, damage can indeed bog down for even moderately skilled players. In reality people have options to get around the defenses of a skilled opponent and for some added complexity GURPS can emulates these options.
First off the players should not pair off with their opponents but should at least double team opponents. In the two-on-two fight this means that one player can force a step back by an enemy then the second player can strike, forcing their opponent to use their base defense.
They could position themselves so one can limit the target's defense (page 390). A side attack get -2 to defense and a rear attack permits no defense roll.
The player can choose to beat down their opponents' shields by striking at their weapons and especially the shield (pages 484, 485).
A skilled player can chose to Feint, basically a contest of weapon skill, and if the player wins the contest the difference is applied as a negative to the opponent's defenses (page 365).
A limb strike to the arm or leg can be effective as there is often little armor in those locations (page 398).
A step and evaluate can be used to gain up to +3 to hit (page 364). While it extends combat in-game time, it takes little time to resolve in table time: the player just declares he is evaluating.
Finally, historical melee combat is as much a physical contest as it is about weapon play. Slams can be used to knock the opponent down to a prone position with all his weapons unready (page 371) and at a -3 to defense. Grappling can be used to immobilize an opponent for a friend.
Look at Knockback on page 278. Basically, for every multiple of ST-2 on damage before armor is subtracted, the opponent is knocked back a hex. More critically it forces them to make a DX, Acrobatics, or Judo roll or fall down.
Don't forget to apply the shock roll once anybody takes damage. For example 4 damage means the person is -4 to all rolls until the next round (page 419). Any damage over HP/2 will force the target to roll vs Health for stunning and knockdown (page 420).
The biggest issue the player will find when using these options isn't their complexity, but figuring out when they can be effectively used. For example, trying to slam a target with a lot more HP than you wouldn't be a good tactic.
Finally, remember the NPCs' morale. Rarely will an intelligent person fight to the death. Some will run away with a just a broken sword arm – and in a two person fight, that will likely cause the other person to run away as well.
Best Answer
The designers saw that exploit coming!
The exploit doesn't work in GURPS 3E nor 4E.
It's easy to miss. (Especially if you want to!)